WASHINGTON — The Republican Party may never have been destined to fall in love with Mitt Romney, but even persuading voters to fall in line behind his candidacy is proving far more taxing than he had once hoped.

The rejection from Republican voters in Colorado, Minnesota and Missouri on Tuesday is more likely to slow, rather than derail, his path to the party's presidential nomination. Yet a new competition with Rick Santorum and a lingering feud with Newt Gingrich will consume the attention of Romney, forcing him to guard his right flank rather than turn his attention to President Barack Obama.

"There's no such thing as coronations in presidential politics," Romney said, trying to remain upbeat Wednesday. He added, "It's not easy to be elected president, and this is a testing."

His latest defeats, along with a palpable dearth of enthusiasm surrounding his campaign, have highlighted several potential weaknesses for him as a general election candidate.

First, Romney clearly does not yet have the support of the Republican base. His advisers had hoped that a unified goal of defeating Obama would rally conservatives behind him, but Santorum's sweep of the three contests this week, along with the continued curiosity about Gingrich, is a sign that the party's core activists are in no hurry to coalesce.

While Romney still holds more tangible advantages than his Republican rivals, it remains an open question whether the traditional metrics of money and organizational muscle still provide the best measure of a candidate.

The contests in Colorado, Minnesota and Missouri provided something of an organic test, where voters made up their minds without the prodding of the negative television commercials that have dominated the race. And by that score, Santorum comfortably outpaced Romney.

But Romney and his advisers are bracing for the prospect of a prolonged nominating contest.

The contours of the next fight started coming into view, with Romney portraying Santorum as an agent of Washington who should be held responsible for the budget deficit from his time in Congress. Romney sought to lump Gingrich and Santorum into the same group who "spent too much, borrowed too much and earmarked too much."

"I don't think we can change Washington if you've been part of the culture of Washington," Romney said, speaking to an audience in Atlanta, where he arrived Wednesday to begin winning over voters for Super Tuesday on March 6, when 11 contests are held.

The timing of Romney's aggressive assault on Washington was hardly ideal. He is scheduled to spend today in the capital, surrounded by lobbyists and other donors who are each asked to raise $10,000 in contributions before attending a policy discussion. His campaign has designated "Industry Finance Chairs" from the energy, defense and financial sectors.

All four remaining Republican presidential candidates will cross paths in Washington on Friday when thousands of party activists gather at the Conservative Political Action Conference. They will make their cases on their electability and their commitment to conservative principles.

.FAST FACTS

NEXT DEBATE: CNN and the Arizona Republican Party will co-host a primary debate among the 2012 presidential candidates at 8 p.m. Feb. 22.

NEXT CONTEST: Maine's caucuses are held over the course of more than a week, with the results to be released Saturday.